Use For HVDC Systems
Many high-voltage direct current systems using submarine power cables are single wire earth return systems. Bipolar systems with both positive and negative cables may also retain a seawater grounding electrode, used when one pole has failed. To avoid electrochemical corrosion, the ground electrodes of such systems are situated apart from the converter stations and not near the transmission cable.
The electrodes can be situated in the sea or on land. Bare copper wires can be used for cathodes, and graphite rods buried in the ground, or titanium grids in the sea are used for anodes. To avoid electrochemical corrosion (and passivation of titanium surfaces) the current density at the surface of the electrodes must be small, and therefore large electrodes are required.
The advantage of such schemes is eliminating the cost of a second conductor, since salt water is an excellent conductor. Some ecologists claim that electrochemical reactions caused by the earth return can affect wildlife. However, these reactions do not occur on very large underwater electrodes.
Read more about this topic: Single-wire Earth Return
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